1. Field of the Invention
The present invention relates to a mobile radio communication system like an automobile telephone system, or a portable telephone system, or a cordless telephone system, or the like. More particularly, the invention relates to a cellular radio communication system which is capable of selectively using either analog mode of digital mode when exchanging radio communication between a base station and a mobile radio station apparatus.
2. Description of the Related Art
Recently, as one of cellular mobile radio communication systems, there is a proposal on such a system which introduces a dual-mode communication by alternatively applying either analog mode or digital mode as required. The proposed dual mode radio communication system comprises a plurality of base stations which are respectively connected to a control station via a plurality of wire circuits and a plurality of mobile radio stations. Each base station has its own radio zone. Each mobile radio station is connected to each base station via a radio circuit in a specific radio zone allocated for each base station.
Either the analog mode or the digital mode is available for such a communication system using radio circuits. When the analog mode is selected, the transmitter angle-modulates a carrier signal in accordance with a voice signal and data and transmits the angle-modulated carrier signal. On the other hand, the receiver initially receives the angle-modulated carrier signal from the transmitter and angle-demodulates the received carrier signal to reproduce the voice signal and the data. On the other hand, when the digital mode is selected, initially, the transmitter digitally encodes voice signal and data, and then modulates a carrier signal by the digitally coded signal before transmitting the modulated carrier signal. The receiver receives the modulated carrier signal from the transmitter, and then demodulates the received carrier signal. Finally, the demodulated signal is decoded before eventually reproducing both the aural signal and the data. When executing a radio communication by the digital mode, a plurality of time slots are transmitted, using a single radio frequency, in accordance with the time-sharing multiplex communication system.
Generally, when executing a radio communication in the analog mode, since the content of speech is transmitted merely by directly modulating a carrier signal by voice signal and data, the content of speech can easily be received by any conventional general-purpose radio receiver. In other words, the content of speech may easily be tapped by any third party, and thus, the security rate is extremely low. Conversely, the time-sharing multiplex communication system is introduced to the digital-mode radio communication. Since all the aural signals and data are digitally coded, even though any third party receives the carrier-wave frequency, he cannot correctly understand the spoken aural signals and data from the digitally coded signal received by his receiver unit. In the other words, the radio communication based on the digital mode provides an extremely high security rate. Based on this reason, it is probable that when the operator desires to engage in a highly confidential communication requiring security, he prefers to apply such a radio communication system capable of securely selecting the digital mode.
Nevertheless, conventionally, any base station unilaterally selects a radio communication system available for either transmission or reception according to the vacancy of available speech channel. In other words, only the base station can unilaterally select either the analog mode or the digital mode. Because of this, even when a user is anxious to telephone-communicate in the digital mode, he can hardly meet his demand. Furthermore, even when the base station selects the digital mode, the user cannot identify that the digital mode has been selected by the base station. As a result, the user cannot identify whether he can actually telephone-communicate with security, or not. In many cases, users are obliged to give up themselves to engage in a highly confidential communication for the security sake.